This invention relates to a temperature controller for use with deep fat fryers or the like and which provide for pulsed operation of the heating element in non-cooking heat demand periods, thereby minimizing temperature overshoot of the cooking oil above the selected temperature. The invention further relates to a temperature controller which monitors for probe failure and for temperature selector switch misfunction or failure and takes protective action in response to those events.
Deep fat fryers are commonly used in restaurants and particularly fast-food type restaurants for preparing foods, such as deep-fat fried potatoes, chicken, fish, onion rings, etc. The deep fat fryers generally comprise a vat containing shortening or other cooking oil and heating means for heating the cooking oil. In some fryers, a gas burner is provided under the vat, and in other fryers electrical resistance heating elements are provided within the vat. Deep fat fryers generally incorporate a thermostat, including a temperature sensor for determining the temperature of the cooking oil, means for selecting a desired temperature for the cooking oil, and heat control means which turn on the heating means when the temperature of the cooking oil falls below the selected temperature.
Many cooking oils, such as shortening, are solid at room temperature, and special care must be taken in operating the fryer to melt the solid shortening. If the heating means is permitted to remain on for a substantial period of time when the shortening is solid, hot spots can develop which may damage the fryer structure and which also tend to scorch the shortening, making it unsuitable for use in cooking food. One solution to this problem is found in U.S. Pat. No. 3,800,779 in which a melt cycle control system is added to the thermostat of the fryer. The melt cycle control system of that patent causes the heating element of the fryer to operate for intermittent periods until liquid shortening reaches a predetermined level in the vat, and thereafter permits the usual thermostat action. After the shortening is melted, it becomes an effective heat sink which protects against isolated hot spots in the fryer structure and also prevents the shortening itself from overheating.
Once the shortening is in its liquid state, the conventional fryer thermostat control calls for continuous operation of the heating means of the fryer when the temperature of the cooking is below the selected temperature. Once the cooking oil reaches the selected temperature (or slightly above the selected temperature), the heating means is turned off and remains off until the temperature of the cooking oil has coasted back to a temperature slightly below the selected temperature. One problem with such a thermostat control is that when the fryer is idling, i.e. not being used for a cooking operation but maintaining the cooking oil at or near the selected temperature in preparation for cooking, full and continuous operation of the heating elements tends to cause the temperature of the cooking oil to overshoot the selected temperature. Thus, the cooking oil may be above the selected cooking temperature when the food item is produced. Also, extra energy is used in heating the cooking oil in this manner when the fryer is idling.